To add a user, you'll add them to the Team section in step 1, #3. Once you add them, you can click the link to "send invitation." Copy and paste the text that comes up and email it to the user. They'll then be able to set up a password and access the account.
If you select the "allow configuration" check box, they will have editing privileges for the WMP. If you do not, they will only be able to view and print the plan.